Flow-pattern-based heat transfer and pressure drop correlations for condensing refrigerants in smooth tubes

Abstract

The phase-out of ozone-depleting refrigerants, such as R-12 and R-22, according to the Montreal Protocol of 1987, has provided the incentive to increase the thermal efficiency of current heating and refrigeration systems. The purpose of this study was to increase the accuracy of the predictions of both the heat transfer and pressure drop correlations for condensing refrigerants in the Intermittent flow regime. This was done utilizing a novel method involving the temporal and spectral analysis of the light intensity of the local flow regime, as seen through a sight glass. An experimental setup was designed, built and commissioned specifically for this purpose using refrigerant R-22 and a smooth tube. It was found that the accuracy of the mean heat transfer coefficient predictions increased substantially compared to other leading correlations,particularly at low mass fluxes. In terms of the pressure drop,the predictions also increased in accuracy, and it was found that the time fraction method allows for continuous predictions over flow regime transitions when using local flow-pattern-based pressure drop models. This was previously not possible.